Systems have been developed that utilize a fluid for generating a force to move or hold an object or for controlling an action. Some of the aforementioned systems include electro-hydraulic servo valves that regulate a given function or variable, such as temperature, speed, pressure, force, displacement or velocity. The foregoing electro-hydraulic servo valve is responsive to an input signal for establishing a pilot fluid control pressure or pressures which is used to control the position of the spool within a valve which in turn directly controls the flow of fluid to a useful load device such as a hydraulic motor. Thus, the control valve or servo valve is a final controlling element which directly changes the value of the controlled variable in a servo control system by varying the rate of flow of some medium. Servo valves are utilized in the diving systems of submarines.
In order to avoid detection by radar and air patrols, a submarine is usually submerged. When a submarine is properly trimmed in the submerged condition, the weight and longitudinal position of the center of gravity of the submarine are equal to those of the water displaced by the submarine. As long as the submarine is moving, underwater the submarine does not have to be perfectly trimmed, diving planes are used to balance moderate positional errors. Diving planes (bell planes and stern planes) are pairs of hydrofoils which extend from the sides of the submarine. The bell planes are mounted on the forward part of the stern of the submarine and the stern planes are mounted at the aft end of the submarine. Each set is mounted on a horizontal strut which may be tilted in either direction from the horizontal to develop a vertical force on the planes and thus on the submarine. Thus, the diving planes are used to control the depth of the submarine. In order to control the submarine's depth the diving planes may be tilted from one angle to another. The tilting of the diving planes requires a large amount of energy, because the diving planes must be moved through a large amount of water. Hence, most submarine diving systems use hydraulic valves so that the diving planes may be more easily moved.
In order to find the submarine, man has developed systems like sonar which enable the operator of the sonar equipment to listen for the submarine. The listening equipment can produce signals which indicate the direction of distant sounds. In order to avoid detection the submarine must run quietly. Low noise hydraulic flow control servo valves were utilized in the prior art for controlling the steering and diving mechanisms of submarines. Two-stage hydraulic valves were used. A single stage valve was not used because a large fast-acting force motor would be required. The aforementioned two-stage valves had a force motor, a flapper valve, a pilot stage and a low noise spool type second stage. An electrical control signal was transmitted to the coils of the force motor to supply power to the force motor. The force motor was coupled to the first stage flapper valve to move the flapper from side to side so that the valve's flow nozzle may be restricted. When the flapper moved close to the nozzle, it causes the pressure behind the nozzle to rise and when the flapper moved away from the nozzle, it causes the pressure behind the nozzle to decrease. Thus, the spool would move away from high pressure to low pressure areas. The force feedback spring or wand was utilized to bring the flapper back to the center position so that in the steady state the main stage spool will stop moving. The force feedback spring is off center in an amount proportional to the input electrical signal. When the foregoing valve was used aboard submarines, the hydraulic fluid flow within the valve would be straightened or made laminar to cut down on the flow noises to make the valve quiet. The aforementioned valve is large and quite expensive because the valve has a pilot stage, a wand, and a nozzle.